In a machine, such as a wheel loader or a tractor, for example, an internal combustion engine supplies power for propelling the machine in the forward or reverse direction and supplies power for further machine implements as well. A transmission is coupled to the engine via a power input shaft and transmits power from the engine to the drivetrain via a power take-off shaft to propel the machine. For this purpose, in practice, diverse types of hydro-mechanical transmissions combining mechanical and hydraulic components are used. Particular types of such transmissions are power split transmissions. Normally, power split transmissions have two distinct branches for power transmission: a mechanical branch and a variable transmissions ratio branch, e.g. a hydrostatic branch comprising a hydrostatic pump connected to a hydrostatic motor or e.g. a CVT (Continuously Variable Transmission) gear set. The power in a hydrostatic branch is usually controlled by the displacement of the hydrostatic pump and/or the hydrostatic motor. In contrast, the power in the mechanical branch is often constant or selectable in fixed steps by means of gear sets and clutches though the power in the mechanical branch varies with the speed of the output shaft of the drive engine. The powers of both branches can be merged again for instance by summation gearboxes, thus allowing a precise control of the machine speed and machine torque. Such transmissions usually permit a forward as well as a reverse motion of the machine.
A simple and reliable exemplary embodiment of such an arrangement is known from the state of the art, exemplarily used in tractors of AGCO Fendt. This embodiment is shown simplified in FIG. 1. The power split transmission of the state of the art comprise a power input shaft for driving a planetary gear set splitting the input power at the power input shaft into a power flow over a hydrostatic power branch driven by the ring gear of the planetary gear set, and into a powerflow over a mechanical power branch connecting the sun gear directly with the power take-off shaft. The powerflow transmitted over the hydrostatic power branch is merged again to the mechanical power branch by a summation gear box having a fixed transmission ratio for driving the power take-off shaft arranged as an extension of the mechanical power branch.
This type of construction of a power split transmission has the drawback that in a reverse driving mode of the machine, hydraulic power is recirculated via the mechanical branch to the planetary gear set. For driving in the reverse direction the rotational direction of the output shaft of the hydrostatic power branch is reversed by swivelling the hydraulic pump to the correspondent other side for instance, thereby changing the rotational direction of the hydraulic motor and the rotational direction of the output shaft of the hydrostatic power branch and thereby forcing the mechanical power branch and the sun gear to rotate in the reverse direction. As the planet carrier is driven by a combustion engine and always turns in the same direction, a forced driving of the sun gear in the reverse direction constitutes power losses. At the same time, a counter-rotated sun gear forces the ring gear to high or even very high revolutions which leads in reverse driving direction to higher speeds in the pump unit and higher flows in the hydrostatic transmission which could result in higher losses in the variable power branch. The increased speed of the pump unit in a reverse driving direction needs to be considered during the design phase of the power split transmission and results in larger hydrostatic pump units. Higher speeds in the pump unit also cause high inertial forces in the hydrostatic pump, especially if the resulting pump-design has to be larger due to the recirculated power. Furthermore, high inertial forces can cause high wear and/or damage in the used drive units of the variable power branch.